Mechanisms of vasoconstrictor responses to KCl in rat isolated perfused tail arteries: interaction with the alpha 2-adrenoceptor agonist UK14304.

The vasoconstriction in rat tail arteries during exposure to 56 mM KCl for 2-5 min consisted of an initial sharp peak followed by a secondary plateau. Both components were reduced by the alpha 1-adrenoceptor antagonists prazosin and WB4010. In arteries from reserpine-pretreated rats, the plateau was markedly reduced and only slightly further attenuated by prazosin, however the initial peak was not reduced but was now not affected by prazosin. Thus, the response to KCl in arteries from normal rats is partly due to release of noradrenaline, and this occurs to a greater extent in the plateau than in the peak component. Addition of UK14304 during the plateau reduced the vasoconstriction in arteries from normal rats; however, in arteries from reserpine-pretreated rats there was increased vasoconstriction. These effects of UK14304 were abolished by idazoxan and were not affected by prazosin, and can be attributed to prejunctional inhibition of noradrenaline release in arteries from normal rats and postjunctional enhancement of vasoconstriction in arteries from reserpine-pretreated rats.